1. Field of the Invention
The present invention relates to an optical amplifier which amplifies signal light in an optical waveguide to which pumping light is supplied, and an optical communication system including the optical amplifier.
2. Related Background Art
An optical communication system transmits large-capacity information with a high speed in such a manner that signal light having a plurality of channels of different wavelengths from each other (WDM: Wavelength Division Multiplexing signal light) propagates through an optical fiber transmission line. The C band (1530 nm to 1565 nm) has been already used as a signal wavelength range in the optical communication system, and the use of the L band (1565 nm to 1625 nm) also is considered. Further, in order to develop further large-capacity of information, and the use of the S band (1460 nm to 1530 nm) is considered as a signal wavelength range.
In the optical communication system, an optical amplifier to amplify signal light is applied. As an optical amplifier which enables to amplify the signal light of the C or L band, utilized is an EDFA (Erbium-Doped Fiber Amplifier) in which an optical amplification fiber (EDF: Erbium-Doped fiber) where Er (erbium) element is doped to its optical waveguide region is applied as an optical amplification medium. The EDFA can amplify signal light of the C or L band which propagates through the Er-doped optical fiber by supplying pumping light (0.98 μm wavelength band or 1.48 μm wavelength band) to the Er-doped optical fiber.
On the other hand, as an optical amplifier which enables to amplify the signal light of the S band, considered is TDFA (Thulium-Doped Fiber Amplifier) in which an optical amplification fiber (TDF: Thulium-Doped Fiber) where Tm (Thulium) element is doped to its optical waveguide region is applied as an optical amplification medium. The TDFA can amplify the signal light of the S band which propagates through the Tm-doped optical fiber by supplying pumping light (1.05 μm wavelength band, 1.4 μm wavelength band, or 1.55 to 1.67 μm wavelength band) to the Tm-doped optical fiber.
In such an optical amplifier, it is important that its gain spectrum is flattened in the wavelength range of signal light to be amplified irrespective of the temperature change. For example, it is required that the gain of an optical amplifier employed in a land-based optical communication system is flat at least within the range of atmosphere temperature: 0° C. to 65° C. irrespective of the temperature. But, the gain spectrum of the optical amplification medium in the optical amplifier is typically not flat in the signal wavelength range, and the gain spectrum may shift toward a short or long wavelength side depending on the temperature. Therefore, it is required that the optical amplifier includes not only a gain equalizing filter for equalizing the amplification gain of signal light in an optical amplification medium but also a temperature compensator for reducing temperature dependency of the gain.